1. Field of the Invention
The present invention relates to an improved directional coupler which is used in the microwave band field, and more particularly, to a loosely coupled type directional coupler constructed by microstrip lines and utilized, for example, as an output monitor of a high power microwave amplifier.
This kind of directional coupler should have a coupling of lower than -20 dB and a satisfactory directivity.
2. Description of the Related Art
Conventional directional couplers are classified into two types, i.e., a branch line coupling type and a distributed coupling type.
The branch line coupling type has a disadvantage in that, when the coupling must be made very small, in order to monitor the output power with a small power loss in the main line, the line width of the microstrip line used as a coupling arm becomes very narrow and is difficult to manufacture.
The distributed coupling type has a disadvantage in that this type of directional coupler has almost no directivity when the coupling is very small.